1. Field of the Invention
The invention relates generally to materials which exhibit nonlinear absorptive properties. More particularly, the invention relates to materials which have high two-photon or higher-order absorptivities and methods for their use.
2. Description of the Related Art
Molecular two-photon absorption was predicted in Göppert-Mayer, M., Ann. Phys. 1931, 9, 273. Upon the invention of pulsed ruby lasers in 1960, experimental observation of two-photon absorption became reality. In years since, multi-photon excitation has found application in biology and optical data storage, as well as in other fields.
Although interest in multi-photon excitation has exploded, there is a paucity of two-photon absorbing dyes with adequately strong two-photon absorption in the correct spectral region for many applications.
There are two key advantages of two-photon (or higher-order) induced processes relative to single-photon induced processes. Whereas single-photon absorption scales linearly with the intensity of the incident radiation, two-photon absorption scales quadratically. Higher-order absorptions will scale with yet a higher power of incident intensity. As a result, it is possible to perform multi-photon processes with three dimensional spatial resolution. Further, because these processes involve the simultaneous absorption of two or more photons, the chromophore is excited with a number of photons whose total energy equals the energy of a multi-photon absorption peak, although each photon individually has insufficient energy to excite the chromophore. Because the exciting light is not attenuated by single-photon absorption in this case, it is possible to excite selectively molecules at a greater depth within a material than would be possible via single-photon excitation by use of a beam that is focused to that depth in the material. These two advantages also apply to, for example, excitation within tissue or other biological materials. In multi-photon lithography or stereolithography, the nonlinear scaling of absorption with intensity can lead to the ability to write features below the diffraction limit of light and the ability to write features in three dimensions, which is also of interest for holography.
The ability to realize many of the possible applications of two-photon or higher-order absorption by molecules rests on the availability of chromophores with large two-photon or higher-order absorption cross sections.